The present invention relates to telecommunication systems in general, and in particular to routing systems such as private branch exchanges.
As the number of telephone service providers increases, most teleconmmunication equipment, such as private branch exchanges (PBX) have the ability to select from several telephone carriers to use when placing a call. The choice of which carrier to use is based on such factors as the capacity of the carrier, the price of a call, desired call quality, etc.
When placing a call on a public switched telephone network a typical PBX uses an automatic route selection (ARS) algorithm to search among trunk groups operated by different public network carriers for an available trunk. Once an available trunk is located, the call is placed by the PBX. If a problem occurs with the call so that the call cannot be completed by the carrier, the carrier plays a message to the caller asking them to hang up and try the call again. When the call is re-attempted, the ARS algorithm will begin searching the available trunk groups and select another trunk on which the call can be placed. Unless network conditions have changed, it is likely that the user will encounter the same error message received on the previous call. Using this approach, the caller may incur the same error message several times in a row regardless of whether the call could have been placed with another carrier having excess capacity.
Given the shortcomings in prior art communication systems, there is a need for a system that can detect and respond to the informational messages produced by the telephone carrier in order to increase the probability of successful completion with which a call can be placed.
The present invention is a telecommunication system that can detect and interpret the informational messages indicating that there is a problem with completing the call through the selected public network carrier. A private branch exchange (PBX) is programmed to implement an automatic route selection algorithm that selects a trunk to carry a telephone call. The automatic route selection (ARS) algorithm monitors the telephone call for an informational message to be transmitted from the carrier indicating that there is a problem with the telephone call placed. If a problem occurs, the automatic route selection releases the first trunk and selects another trunk from a set of remaining trunk groups to ensure that a call is not rerouted over the trunk initially selected. On an ISDN line, the error message is detected on the D channel. On an analog line, the private branch exchange is programmed to detect the special information tones (SIT) or error tones that precedes a spoken error message.